Heat-curable epoxy resin compositions have long been used as adhesives in bodyshell construction and in reinforcing elements or structural foams.
An important field of use of heat-curable epoxy resin compositions is thus in motor vehicle construction, and more particularly in the adhesive bonding or foam filling of cavities in bodyshell construction. In both cases, after the epoxy resin composition has been applied, the body is heated in a cathodic electrocoating furnace, which also cures and, if appropriate, foams the heat-curable epoxy resin composition.
Reinforcing elements comprising a heat-curable epoxy resin composition which forms a structural foam under the reaction of heat are known, for example, by the SikaReinforcer® trade name. U.S. Pat. No. 6,387,470 B1 discloses, for example, a heat-curable and foamable sealing compound which comprises a mixture of liquid epoxy resin and solid epoxy resin in the presence of 5-20% by weight of a thermoplastic such as polystyrene and 30-45% by weight of a thermoplastic elastomer such as SBS block copolymer. The mechanical properties, for example, the glass transition temperature, and adhesion properties of such compositions are, however, greatly worsened due to the high proportion of SBS block copolymer.
In order that rapid curing is possible, not only heat-activatable hardeners for epoxy resins but typically also accelerators are used. A significant category of accelerators is that of ureas. Heat-curable epoxy resin compositions which may comprise ureas as accelerators are known, for example, from International Publication Nos. WO2004/106402 A2 and WO2004/055092 A1.
Epoxy resin compositions are usually foamed by means of chemical blowing agents, generally organic azo compounds, which decompose when heated, or by means of expandable hollow polymer spheres filled with a low-boiling organic compound. The former afford primarily nitrogen through the decomposition reaction, forming toxic ammonia as a by-product according to the conditions. Expandable hollow polymer spheres again are filled with inflammable hydrocarbons, and therefore storage and transport are subject to particular regulations. Such chemical blowing agents are known by the Expancel® trade name from Akzo Nobel or Celogen® trade name from Chemtura or Luvopor® trade name from Lehmann & Voss, Germany.
The literature additionally also describes carbon dioxide, which is generated in a polymer matrix by reaction of chalk (CaCO3) with phosphoric acid. However, such a process is possible only with two-component epoxy resin compositions and in that respect is not an option for (one-component) heat-curable epoxy resin compositions.